Blender 2.8 compositor isn't saving me any time?
No adaptive sampling in Cycles in Blender versions prior to 2.83
Cycles used lack adaptive sampling functionality in versions lower than 2.83 so no matter what, the pixels being rendered used to get sampled as many times as there were samples defined in the render settings.
Mask with Holdout shader
You could make a plane, parent it to the camera(Ctlr+p) using Without Inverse option so it snaps to the camera and is oriented to it's local space, move the plane and cut a hole in it so it's sort of a mask. If you assign Holdout shader to it it will get rendered as transparency (transparent film enabled) and it will still get sampled, however those samples are going to be as easy to calculate as possible, so it will speed up the process greatly. You can make any shape of the mask just by modelling the hole the way you need it to be, however if it covers anything that you need in the render(like shadows for example) that will become invisible, so you need to be careful about what you mask.

You can see the masked areas outside the center of the image render a lot faster:

Adaptive sampling is added since Blender 2.83
From version 2.83 adaptive sampling is finally added and if enabled will render transparent areas a lot faster.